The present invention claims the benefit of Korean Patent Application No. P 2000-45924 filed in Korea on Aug. 8, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display device (hereinafter, referred to as LCD), and relates more particularly to a multidomain LCD having a simplified manufacturing process, an improved viewing angle and increased transmissivity.
2. Description of Related Art
When manufacturing LCD devices, the aim is to provide a vivid viewing screen that does not fatigue a user""s eyes.
A LCD is comprised of two substrates that face each other with liquid crystal injected between the two substrates. A LCD generally uses a liquid crystal having a twisted nematic (hereinafter, referred to as TN) mode. The liquid crystal has different refractive anisotropy between the light propagating in a direction of the longitudinal axes of liquid crystal molecules (n11) and the light propagating in a direction perpendicular to the direction of the longitudinal axes (nxe2x8axa5). This difference results in a viewing angle that is substantially narrow.
Many solutions for addressing the problem of a narrow viewing angle in a LCD have been proposed, examples of which are as follows: a film compensated mode in which a compensating film is provided to compensate for the viewing angle; a multidomain mode in which a pixel is divided into a plurality of domains, so that the main viewing angle on the respective domains is different from each other, thereby increasing the effective viewing angle; a vertical alignment mode in which the alignment of the liquid crystal is in a vertical direction when a voltage is not applied; and an in-plane switching mode in which the liquid crystal molecules are rotated horizontally by using a parallel electric field formed by the two electrodes arranged on a single substrate.
The liquid crystal cells in which the multidomain mode is applied are a domain divided twisted nematic (DDTN) liquid crystal cell, a two-domain twisted nematic (TDTN) liquid crystal cell, a complementary twisted nematic (CTN) liquid crystal cell and a four-domain twisted nematic (FDTN) liquid crystal cell. In the case of a FDTN, the four domains are divided on the unit pixel such that the gray inversion in four directions compensate for each other, thereby providing a broader viewing angle than that of a two-domain liquid crystal cell.
The above-mentioned multidomain modes are achieved by mechanical rubbing or irradiating light on the alignment layers of the two substrates, respectively, to control both a pretilt angle and a pretilt direction so as to control the direction of the liquid crystal. In the case of mechanical rubbing, a photolithography is carried out several times. Whereas in the case of irradiating the light, the light irradiating process has to be carried out several times for each domain using a mask. Both of these process require a complicated manufacturing process.
However, the alignment treatment has not been widely used in recent years. Another method has been developed in which a slit in the electrode formed on the substrate distorts the electric field applied to the liquid crystal layer, such that the direction of the liquid crystal molecules are positioned in a desired direction. Examples of the above method are patterned vertical alignment (hereinafter, referred to as PVA) and lateral field induced vertical alignment (hereinafter, referred to as LFIVA).
The PVA is carried out with a plurality of slits formed by etching the transparent electrodes on both the upper and lower substrates, and thus, the azimuth angle of the alignment of the liquid crystal is determined by a lateral electric field generated at the time when a voltage is applied to pixels. The LFIVA is carried out with a plurality of slits formed in the pixel electrode and with the common electrode rubbed in the longitudinal directions of the slits, such that the electric field formed has horizontal components as well as vertical components.
The formation of the slits requires a patterning process comprising the steps of: forming an electrode film on the whole surface of the substrate; forming a photoresist film on the electrode film; exposing the photoresist film using a mask; etching the exposed photoresist film to form a pattern; and patterning the electrode film by using the patterned photoresist as a mask.
Vertical alignment is made in the LIFVA so that the directions of the longitudinal axes of the liquid crystal molecules are aligned vertically to the substrate surface. In more detail, the liquid crystal having a negative dielectric anisotropy is injected, so that the longitudinal axes of the liquid crystal molecules are disposed vertically on the plane of the alignment layer when no voltage is applied. On the other hand, the liquid crystal molecules move from being vertically disposed on the plane of the alignment layer to being horizontally disposed on the plane of the alignment layer when a voltage over a threshold voltage is applied.
FIG. 1 is a sectional view of related art PVA, and FIG. 2 is a sectional view of related art LFIVA.
As shown in FIG. 1, the conventional PVA comprises first and second substrates 11 and 15 facing each other with a liquid crystal layer 10 formed between the first substrate 11 and second substrate 15. The first substrate 11 has a black matrix (not shown) for preventing light-leakage, a color filter layer 13 between the black matrixes and a common electrode 14 having a plurality of slits on the color filter layer 13. The second substrate 15 has a plurality of data lines and gate lines (not shown) arranged perpendicularly to one another that define a plurality of pixel areas. Each of the pixel areas has a thin film transistor (not shown) with a gate electrode, a gate insulation film 16, a semiconductor layer, a source electrode and a drain electrode. At the cross point of the data lines and gate lines, a protective film 17 is formed on the whole surface of the thin film transistor, and a pixel electrode 18 is connected to the drain electrode of the thin film transistor on the protective film 17. The pixel electrode 18 has a plurality of slits 19.
Specifically, the plurality of slits in the common electrode 14 and the pixel electrode 18, each respectively require a patterning process.
In more detail, the first substrate 11 and second substrate 15 are first prepared. The gate lines and the gate electrode (not shown) are formed on the second substrate 15. Then, the gate insulation film 16 is formed on the gate electrode of the second substrate 15. Thereafter, a semiconductor layer (not shown) is formed on the gate insulation film 16 and subsequently the data lines are formed perpendicularly to the gate lines. At the same time, the source/drain electrodes (not shown) are formed on the semiconductor layer.
The gate lines, gate electrode, data lines and source/drain electrodes are formed of a metal having a low resistance, such as Cu, Al and Mo or an Al alloy, by sputtering and patterning. The gate insulation film 16 is formed of an inorganic material having an excellent adhesion with the above metal and a high insulation internal pressure, such as SiNx, SiOx, etc., by plasma enhanced chemical vapor deposition (PECVD).
Next, the protective film 17 of SiNx, SiOx or Benzocyclobutene (BCB) having a low dielectric constant is formed on the whole surface of the laminated layer. Then, the protective film 17 is selectively removed to expose a predetermined portion of the drain electrode to form a contact hole. Subsequently, the pixel electrode 18 made of a transparent conductive material, is formed on the protective film 17 and is electrically connected to the drain electrode through the contact hole.
Thereafter, the photoresist (not shown) is applied on the pixel electrode 18 and is patterned using photolithography. Then, the pixel electrode 18 is selectively etched using the photoresist pattern as a mask to form a plurality of slits 19 in the pixel electrode 18.
The black matrix and the color filter layer 13 are formed on the first substrate 11. Then, a transparent conductive material is deposited to thereby form the common electrode 14. In the same manner as described above, photolithography is used to form slits 19 in the common electrode 14.
The first substrate 11 and second substrate 15 are attached facing each other. Then, the liquid crystal 10 having a negative dielectric anisotropy and containing a chiral dopant is injected and the space between the first substrate 11 and second substrate 15 is sealed, thereby completing the LCD.
The common electrode 14 and the pixel electrode 18, which apply a voltage to the liquid crystal 10, are formed of indium tin oxide (hereinafter, referred to as ITO) in which 5% tin oxide is mixed. The ITO can be spray deposited on a glass substrate, deposited by vacuum deposition when the material is formed on the substrate in a vacuum vessel, or deposited by high-frequency sputtering when the material is discharged in a gas at a low pressure.
The angle (between the slit edge and the surface of the adjacent substrate edge) of the slit 19 is 90xc2x0, as shown in FIG. 1. One of the plurality of slits 19 is formed in the common electrode 14, and two are formed in the pixel electrode 18, within the unit pixel area. Since the alignment direction of the liquid crystal molecules is determined based upon the application of the electric field from the slits 19, the vertical alignment layer is applied without a rubbing process.
If a voltage over a threshold voltage is applied to the PVA-structured LCD formed as discussed above, the slit 19 cause the liquid crystal molecules initially aligned in a vertical direction to become parallel to the planar direction of the pixel electrode 18 centered around the slits 19. As a result, the directions of the main viewing angle are the same in domains A and C, and the directions of the main viewing angle are the same in the domains B and D. In other words, the unit pixel area is divided into two domains, thereby embodying a two-domain LCD.
On the other hand, as shown in FIG. 2, the LFIVA is structured in such a fashion that a plurality of slits 29 are formed only in a pixel electrode 28 on a second substrate 25. A color filter layer 23 and common electrode 24 are formed on the first substrate 21. An alignment layer 30 is applied on the common electrode 24 and are rubbed in a direction of the Y axis, i.e., the longitudinal axis of the slit 29, thereby determining an alignment direction for the liquid crystal molecules 20.
In the LFIVA structure, when no voltage is applied, the longitudinal axes of the liquid crystal molecules 20 are arranged perpendicular to the plane of the alignment layer 30. When the voltage over a threshold voltage is applied, the longitudinal axes of the liquid crystal molecules over the second substrate 25 are disposed toward the plane of the alignment layer, and the liquid crystal molecules over the first substrate 21 are aligned parallel to the rubbing direction as a result of the induced lateral electric field between the slits 29 in the pixel electrode 28. In other words, the LFIVA structure creates both the vertical electric field between electrodes 24 and 28 and the lateral electric field caused by the slits 29 in the second substrate 25 are in vertical relation with each other, thereby forming two domains centered around respective slits 29 within a unit pixel.
Therefore, the liquid crystal alignment structure that is twisted in a clockwise direction and the liquid crystal alignment structure that is twisted in a counterclockwise direction are formed with respect to the slit 29 between the two structures, thereby exhibiting the two-directional main viewing angle within the unit pixel. As a result, the gray inversion in the two directions compensates for each other, thereby exhibiting excellent transmissivity, response time and viewing angle compared to a single domain.
However, the conventionally developed multidomain LCDs have several problems.
First, a separate patterning process is required to form the slits in each of the transparent conductive layers on the first and second substrates in PVA to obtain a multidomain LCD. Therefore the manufacturing process becomes more complicated.
Secondly, the LFIVA requires mechanical rubbing of the alignment layer on the first substrate, which may generate dust or static electricity and thus degrade the quality of the device.
Third, the LFIVA does not sufficiently twist the liquid crystal molecules toward the direction of the slits, such that the transmissivity decreases in accordance with the angle of the polarization axis.
Finally, the LFIVA is a two-domain structure dependent upon the viewing direction, such that the color and contrast ratio vary depending upon the viewing direction.
The present invention is directed to a multidomain liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
It is an object of the present invention to provide a multidomain liquid crystal display device that is capable of appropriately adjusting the angle of the boundary line of a slit formed on one substrate and the pitch of liquid crystal to obtain multidomain for a main viewing angle on each of domains, resulting in a simplified manufacturing process, and improved viewing angle and transmissivity.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a multidomain LCD includes: first and second substrates, the first substrate having a plurality of pixel areas, a transparent conductive layer on each pixel area of the first substrate and having at least one or more slits inclined at a prescribed angle with respect to a boundary of the pixel area, and a liquid crystal layer between the first and second substrates.
In another aspect, multidomain LCD includes first and second substrates, a plurality of data lines and a plurality of gate lines arranged substantially perpendicular to one another on the first substrate to define a plurality of pixel areas, a pixel electrode on each pixel area on the first substrate and having at least one or more slits inclined at a prescribed angle with respect to a boundary of a pixel area, first and second vertical alignment layers on the first substrate respectively, and the second substrate, and a liquid crystal layer between the first and second substrates.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.